Blower Door Test
A diagnostic test that measures the airtightness of a building envelope using a calibrated fan mounted in a door frame. The result is expressed as the n50 value — the number of air changes per hour at a pressure difference of 50 pascals.
A blower door test is the single most reliable way to find out whether a building envelope actually performs the way it was drawn. Instead of trusting the details on paper, it puts the finished construction under measured pressure and reveals every path that air can take through it. For passive houses the test is not optional — it is the evidence that the airtightness layer works.
What is a blower door test and how does it work?
A calibrated fan is sealed into an external door opening with an adjustable panel. The fan either extracts air to depressurise the building or pushes air in to pressurise it, until a stable pressure difference of 50 pascals between inside and outside is reached — roughly the load a steady wind would place on the facade.
At that pressure the instrument records how much air the fan has to move to hold the difference. If the envelope were perfect the fan would barely turn; in reality it must continuously replace the air escaping through gaps. That airflow, divided by the internal volume of the building, gives the n50 value in air changes per hour.
The test does more than produce a number. Under depressurisation, air is drawn inward through every leak, so the team can walk the building with a smoke pen or a thermal camera and locate each weakness precisely — a service penetration, a badly taped membrane joint, a window reveal. This is why the most useful test is run while the airtight layer is still accessible for sealing rather than buried behind finishes.
What do the results mean for a passive house?
The n50 value places a building on a clear scale. A leaky older house can exceed ten air changes per hour; a competent modern build lands near one; a certified passive house must reach 0.6 or lower. Each step down represents a large reduction in the heat carried away by uncontrolled airflow.
| Building type | Typical n50 (/h) | Interpretation |
|---|---|---|
| Older existing house | 5 to 15 | Constant draughts, high heat loss |
| Standard new build | 2 to 4 | Meets code, still leaky |
| Low-energy house | 1 to 1.5 | Good practice |
| Certified passive house | 0.6 or lower | Required threshold |
Reaching 0.6 is not achieved by a single product but by a continuous, deliberately designed airtight layer that wraps the whole envelope and is protected at every junction and penetration. The blower door test is what confirms that this continuity survived the reality of the building site.
Crucially, airtightness only makes sense alongside mechanical ventilation with heat recovery. Sealing the envelope removes random air exchange; the ventilation system then supplies fresh, filtered air on purpose and recovers most of the heat from the air it expels. Together they deliver comfortable, healthy indoor air at a fraction of the energy a draughty house would waste.
Frequently asked questions
- When during construction should a blower door test be performed?
- The most valuable test happens once the airtight layer is complete but still accessible — before plasterboard, screeds and finishes are installed. Leaks found at this stage can be sealed cheaply. A second, formal test is then carried out on the finished building to certify the final n50 value.
- What n50 value does a passive house require?
- The Passive House standard requires an air change rate of n50 ≤ 0.6 per hour. This is roughly ten times tighter than a conventional new-build, and it is a hard pass/fail threshold for certification rather than a target average.
- Does an airtight house feel stuffy?
- No. Airtightness stops uncontrolled draughts, but fresh air is supplied deliberately through mechanical ventilation with heat recovery. Occupants get a constant, filtered supply of fresh air without the heat losses caused by random leakage.